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        class Test extends React.Component{
            constructor(...args){
                super(...args)
            }
            render(){
                return <h1>123</h1>
            }
        }
        class Item extends React.Component{
            constructor(...args){
                super(...args)
            }
            render(){
                return <li>{this.props.str}</li>
            }
        }
        class List extends React.Component{
            constructor(...args){
                super(...args)
            }
            render(){
                let aItems=[]
                for(let i=0;i<this.props.arr.length;i++){
                    aItems.push(<Item key={i} str={this.props.arr[i]}></Item>)
                }
                return  <ul>
                    {aItems} 
                </ul>
            }
        }
        window.onload=function() {
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            ReactDOM.render(
                <List arr={['a','b','v']}></List>,
                oDiv
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